1. Field of the Invention
The invention relates to pulse generation circuits and, more particularly, to pulse generation circuits having output pulses that are synchronized to a reference signal.
2. Description of the Prior Art
There are many applications for pulse generation circuits that generate output pulses that are synchronized to a reference signal. Typical applications for such circuits include digital data communications systems and digital data recording systems. For example, many digital communications systems transmit digital data in discrete "packets" or "bursts" that are serially arranged in groups to form transmission frames. Use of such frames allows for utilization of conventional time division multiple access (TDMA) techniques to transmit the bursts between users of the communications system. Examples of such communications systems include satellite communications systems, packet radio systems, local area network (LAN) systems and the like. A receiver of such burst transmissions must periodically receive an accurate timing synchronization pulse to indicate the beginning of each frame of bursts. Using this synchronization pulse, a receiver can de-multiplex and demodulate the bursts within each frame.
Additionally, many digital communication systems, such as high definition television (HDTV) broadcasts, transmit interleaved data to promote robust transmission characteristics. To permit the interleaved data to be de-interleaved within an HDTV receiver, the receiver generates periodic timing synchronization pulses from the data. Such timing synchronization pulses mark the beginning of an interleaved data set, i.e., a block of digital data having its bits interleaved with one another. Each synchronization pulse provides a reference location within the transmission from which the interleaved data is de-interleaved in a conventional manner.
Also, there are many systems other than communications systems, e.g., digital video tape recorders and digital audio tape recorders, that record data using interleaving techniques. Consequently, to facilitate de-interleaving of the recorded data, these recorders generate, during playback, timing synchronization pulses from the recorded data. As with the communications system, the synchronization pulses indicate the beginning of each interleaved data set.
The pulse generation circuits used to generate synchronization pulses in both communication systems and non-communication systems are substantially similar. Therefore, for simplicity, the remaining discussion will address pulse generation circuits in the context of those used in communications systems.
Typically and with respect to communications, a synchronization pulse is transmitted as a "preamble" appended to either a frame or an interleaved data set. Such a preamble is typically a fixed pattern of bits (preamble pattern) that are periodically transmitted at the beginning of each frame or interleaved data set. As the receiver of the transmission receives the data bits carried by the transmission, circuitry in the receiver searches all these received bits to determine a match between the received bits and the preamble pattern. This circuitry is known as a preamble correlator. Whenever a match occurs between the received data bits and a preamble pattern stored in the preamble correlator, the correlator generates a synchronization pulse. This synchronization pulse marks the beginning of an interleaved data set or a transmission frame. Using this synchronization pulse, a conventional de-interleaver circuit de-interleaves the data or a conventional de-multiplexing circuit facilitates appropriate demodulation of the bursts within a frame.
More specifically, to produce a synchronization pulse, most conventional preamble correlators typically must match every bit in a received preamble to the stored preamble pattern. However, in some correlators, to produce a synchronization pulse, only M-bits in the received preamble need match M out of N-bits in the preamble pattern. In such correlators, each matching pair of bits is assigned a value of one; each non-matching pair of bits is assigned a value of zero. The assigned values are then summed to form a summation value. If the summation value is greater than a pre-established threshold value, i.e., a value equivalent to a match of M bits, the correlator generates a synchronization pulse. On the other hand, if the summation value does not exceed the threshold, the correlator does not produce a pulse. In this manner, the correlator produces a synchronization pulse for each instance that M-bits in the received preamble match M out of N-bits in the stored preamble pattern.
In practice, due to a number of factors, correlators, at times, generate errant synchronization pulses (false positive correlations) and, also, miss the occurrence of a preamble in the digital data (false negative correlations). A correlator generates false positive correlations when either payload data within a frame contains a sequence of bits that match the preamble pattern or when channel noise correlates with the stored preamble pattern. Moreover, a correlator produces false negative correlations when the channel noise masks the preamble bits to an extent that the correlator does not recognize an occurrence of the preamble pattern within the digital data. Consequently, such false positive and false negative correlations cause the synchronization pulses to be aperiodic and, as such, can be detrimental to the operation of de-multiplexing and de-interleaving circuits that utilize the synchronization pulses.
To alleviate the impact of false or missing synchronization pulses upon the demodulation circuitry (including de-interleaver circuits, de-multiplexing circuits and any other circuits within the receiver that utilize the synchronization pulses), many synchronization pulse generators contain so-called flywheel circuits. These flywheel circuits produce synchronization pulses at locations where a pulse should be located even though the preamble correlator has not generated such a pulse. In other words, from previous periodic occurrences of the synchronization pulses, the flywheel circuit extrapolates the position by the correlator. The flywheel circuit then generates a synchronization pulse at that position to replace the missing pulse. Transient reduction of signal strength due to multipath or excessive noise conditions, is typically responsible for the temporary loss or drop-out of synchronization pulses, i.e., false negative correlations. Therefore, by using flywheel circuits to replace the missing pulses, these transient pulse losses do not appreciably effect the performance of demodulator circuits within the receiver.
Additionally, flywheel circuits are typically designed to ignore the intermittent occurrence of false positive correlations that produce synchronization pulses between accurate synchronization pulses. Such flywheel circuits typically only accept as accurate the synchronization pulses that occur at a specific, periodic time interval, i.e., at an interval equivalent to that of the preamble occurrences in the received transmission. All other occurrences of synchronization pulses, i.e., all false positive correlations, are ignored by the flywheel circuit and, as such, do not appreciably effect the pulse production by the flywheel circuit.
One such flywheel circuit is disclosed in U.S. Pat. No 5,058,106 (issued Oct. 15, 1991 to G. B. Cole-- the '106 Cole patent). This patent teaches using discrete digital components to form a flywheel circuit that generates synchronization pulses or strobes. These strobes are used for time slot synchronization in a TDMA digital communications system. The flywheel circuit synchronizes to nominally periodic input synchronization pulses generated by a correlator and produces replacement synchronization strobes during drop-outs of the input synchronization pulses. While synchronized, all aperiodic input synchronization pulses, i.e., false positive correlations, are ignored by the flywheel circuit. After a specific time interval during which no input synchronization pulses are synchronized to the replacement strobes, the flywheel circuit re-synchronizes to the input synchronization pulses. The circuit taught in the '106 Cole patent is "hardwired" to have pre-set operational parameters. These parameters include: (1) the number of input synchronization pulses that must be missing before the circuit attempts to re-synchronize, (2) the number of input synchronization pulses that must be missing before the circuit ceases producing replacement synchronization strobes, and (3) the number of input synchronization pulses that must periodically occur before re-synchronization is complete. Each of these parameters is controlled by a specific number of flip-flop circuits used to implement the circuit. Thus, if the parameters must be altered or a different application for the circuit is found that requires slightly different parameters, complete redesign of the flywheel circuit is necessary. Additionally, implementation of this flywheel circuit using discrete flip-flop circuits is complex and costly.
Another flywheel circuit is disclosed in U.S. Pat. No. 4,059,812 (issued Nov. 22, 1977 to S. A. Procter-- the '812 Procter patent). This patent teaches a flywheel circuit that includes a conventional analog tank circuit. The tank circuit is connected to a preamble correlator which supplies nominally periodic input synchronization pulses. The tank circuit is designed to resonate (oscillate) at the frequency of these periodic input synchronization pulses. As such, the tank circuit produces a sinusoidal signal that is synchronized to the periodic synchronization pulses from the correlator. A threshold detector converts the sinusoidal signal into a pulsed output signal that is also synchronized to the input synchronization pulses. If the correlator intermittently produces pulses corresponding to false positive correlations, operation of the tank circuit is unaffected. Moreover, if the correlator subsequently ceases to produce input synchronization pulses, the tank circuit is designed such that the sinusoidal signal is slowly attenuated over time. Thus, for a pre-determined time period equivalent to a time constant of the tank circuit, the pulsed output signal from the threshold detector continues during temporary interruption of the input synchronization pulses. In this manner, transient loss of the input synchronization pulses does not effect the output signal of the flywheel circuit. Thus, a continuous series of pulses is produced by the flywheel circuit even though the input synchronization pulses from the correlator may be temporarily interrupted or intermittently aperiodic.
Unfortunately, this tank circuit-based flywheel circuit must be designed to resonate in response to a specific input synchronization pulse periodicity. Additionally, the duration during which the flywheel circuit is capable of producing pulses without an input signal is pre-established by the tank circuit resonance characteristics. As such, the circuit is designed with a specific application in mind and is not alterable without redesigning and physically altering the circuit.
Therefore, a need exists in the art for synchronous pulse generation circuits, and in particular, flywheel circuits that are flexible, i.e., they contain programmable parameters that govern the operation of the circuit. Such a programmable flywheel circuit would permit a single circuit design to be useful in many different applications. Additionally, such a circuit would permit the parameters to be altered during circuit operation such that the circuit can be dynamically optimized in view of present environmental characteristics such as transmission noise.